The general forms of most metal detectors are either hand-held battery-operated units, conveyor-mounted units, or vehicle-mounted units. Examples of hand-held products include detectors used to locate gold, explosive land mines or ordnance, coins and treasure. Examples of conveyor-mounted units include fine gold detectors in ore mining operations, and an example of a vehicle-mounted unit includes a unit to locate buried land mines.
These metal detectors usually, but not necessarily, consist of transmit electronics generating a repeating transmit signal cycle of a fundamental period, which is applied to an inductor, for example a transmit coil, which transmits a resulting varying magnetic field, sometimes referred to as a transmit magnetic field.
These metal detectors may also contain receive electronics that processes a receive signal from a measured receive magnetic field, during one or more receive periods during the repeating transmit signal cycle, to produce an indicator output signal, the indicator output signal at least indicating the presence of at least a metal target within the influence of the transmit magnetic field.
During the processing of the receive signal, the receive signal is either sampled, or synchronously demodulated, to produce the indicator output signal.
Time domain metal detectors typically include pulse-induction (“PI”) or pulse-induction like metal detectors, and rectangular current-pulse metal detectors, wherein the processing of the receive signal includes sampling of the receive signal and/or synchronous demodulation over selected periods, which may include gain weighting of the samples or synchronous demodulation periods.
Time domain metal detectors that require one or more transmit/receive switches (T/R switches) usually have at least a high-voltage period and at least a low-voltage period within a repeating transmit signal cycle. Usually one or more T/R switches operate to have the T/R switches disconnect (open circuit) the receive electronics from the coil during a high-voltage period when the voltage across the coil is relatively high (100˜200V, for example), and connect (“short circuit”) the receive electronics to the coil during a low-voltage period when the voltage is relatively low (0˜2V, for example). In a commonly known arrangement of a PI detector, the low-voltage period is a non-transmitting zero current receive period following a high-voltage “back-emf” period. However, it should be understood that during this non-transmitting zero current receive period, although relatively very small currents are induced in the transmit coil owing to signals received from the environment, nevertheless, a person skilled in the art would consider this a zero transmit current and receiving period.
A T/R switch is used to protect one or more preamplifiers of the receive electronics of a metal detector from a high voltage during the high-voltage period. A T/R switch can also improve signal-to-noise ratio of the receive signal of a PI detector compared to using the damping resistor to isolate the high voltage back-emf from the preamplifier input. While T/R switches are commonly used in a circuit of a metal detector using a same coil for transmission and reception, T/R switches can also be used in a circuit of a metal detector using separate coils for transmission and reception.
Time domain metal detectors that employ T/R switches usually synchronously demodulate (or sample) a receive signal during a low-voltage period after a delay following the transition of a high voltage-period to the low-voltage period. The outputs of the synchronous demodulation are then averaged, usually by means of low-pass filtering, and may also be high-pass filtered. These averaged or filtered signals are then used for further processing, for example, for setting ground balance or for discrimination. The processed signal can then be used to feed an indicator output, for example, an audio response for indicating a target within an influence zone of the transmit magnetic field.
However, it was discovered that known arrangements of one or more T/R switches within a detector create some issues. The present invention addresses one or more of the issues, and improves the performance of a detector.